1. Field of the Invention
The invention relates to a method and an apparatus for exposing a semiconductor substrate, and more particularly, to an improved method of exposing substrates during the fabrication of semiconductor devices and corresponding apparatus for performing the improved method.
2. Description of the Related Art
Conventional processes for fabricating semiconductor devices include a plurality of repeated deposition processes and patterning processes. The deposition processes typically form one or more new layers of material on a semiconductor substrate such as a silicon wafer, with subsequent patterning processes removing portions of the layer(s) to form a pattern of remaining material on the substrate. Conventional patterning processes include both a photo process and a lithography process. The photo processes include forming a photosensitive film, e.g., positive or negative photoresist, on the substrate, exposing regions of the photosensitive film and developing process the exposed film form a photoresist pattern that reveals portions of the underlying material layer. The exposed portions of the underlying material layer may then be partially or completely removed using the photoresist pattern as an etch mask during the subsequent lithography process.
The photo processes are typically performed in an exposing apparatus that includes an exposing unit in which the photosensitive film is exposed, a developing apparatus that includes a developing unit for selectively removing portions of the exposed photosensitive film, and a plurality of treatment units for performing various operations on the substrate before and/or after the substrate has been processed in the exposing unit. In a conventional exposing unit, portions of the photoresist layer formed on the substrate is irradiated through a photomask or reticle using light of appropriate wavelength and energy to expose the photoresist. This partially exposed photoresist layer is then developed using an appropriate developing solution for the particular photoresist being used, thereby removing regions of the photoresist layer to form a photoresist pattern that exposes regions of the material layer.
The treatment units may include one or more pre-treatment units for treatment the substrate prior to the exposing process, one or more post-treatment units for treatment the substrate subsequent to the exposing process and one or more outer transfer units for transferring the substrate between the pre-treatment unit(s) and the post-treatment unit(s).
Conventional exposing apparatus include an exposing unit for directing light onto regions of the substrate, an aligning apparatus for determining an exposing position of the substrate and an inner transfer unit for transferring the substrate between the aligning apparatus and the exposing apparatus. Sequential movement exposing apparatus have been utilized for producing highly integrated semiconductor devices. Typically, within a sequential movement exposing apparatus, the photomask or reticle will be fixed relative to the light source while the substrate is repeatedly repositioned relative to the photomask in a series of steps. In this manner the pattern or image provided on the photomask, typically provided at 5× or 10× the size of the final pattern, is typically reduced and sequentially imaged onto a number of adjacent regions on the substrate surface, with each region constituting one “shot” of the photomask image onto the photoresist layer.
The accumulated shots of the photomask pattern onto the photoresist layer produce an exposed photoresist layer, that when developed, produces a photoresist pattern on the substrate that reproduces the pattern originally provided on the photomask. Reducing projection exposing apparatus and reducing scanning exposing apparatus have been utilized within a variety of sequential movement exposing apparatus.
Reducing projection exposing apparatus are may be configured to irradiate the region of the photoresist film corresponding to a single shot with light that passes through the photomask at one time. The substrate may then be repositioned relative to the photomask and the exposure repeated, in its entirety, to complete the next sequential shot. This operation is then typically repeated, step-by-step, to expose the majority of the photoresist film formed on the substrate surface in a series of coordinated shots. Depending on the relative dimensions of the substrate and the shot size, a portion of the peripheral regions of the substrate are typically not exposed, thereby avoiding less productive shots and increasing the number of substrates that can be processed through the exposing apparatus in a given period of time. Accordingly, these apparatus are commonly referred to as a “stepper” or “stepper module,” with this particular mode of operation commonly being referred to as a “step-and-repeat” mode.
In contrast, reducing scanning exposing apparatus scan light across the photomask and are, therefore, irradiating only a portion of a whole “shot” at any given time during the exposure process. A shot is completed after the entire photomask has been scanned onto the photoresist layer, after which the wafer is repositioned and the next shot initiated by beginning the next scan of the photomask. This process of scanning of light onto a corresponding portion of the photomask and, consequently onto a portion of the photoresist that comprises a single shot is then repeated step-by-step to expose the majority of the photoresist film formed on the substrate surface in a series of coordinated shots. Again, depending on the relative dimensions of the substrate and the shot size, a portion of the peripheral regions of the substrate are typically not exposed, thereby avoiding less productive shots and increasing the number of substrates that can be processed through the exposing apparatus in a given period of time. Accordingly, this type of apparatus is commonly referred to as a “scanner” or “scanner module,” with this particular mode of operation commonly being referred to as a “step-and-scan” mode.
When a silicon wafer substrate is processed in conventional exposing apparatus as detailed above, a photoresist composition is typically applied to the wafer as a layer using a spin coater to distribute a centrally deposited volume of the photoresist composition across the surface of the wafer to form a layer of photoresist. The photoresist coated wafer is then subjected to an aligning process utilizing one or more “flat” regions provided at the edge of the wafer during initial wafer fabrication and/or other alignment structures provided on the wafer surface during previous processing steps. An outer transfer unit may then be utilized to transfer the wafer to an exposing apparatus utilizing a stepper or scanner unit for exposing a portion of the photoresist layer.
Exposing the photoresist to light alters the chemical composition of the exposed regions of the photoresist layer by either forming larger polymers from smaller components present in the original photoresist layer or more typically in highly integrated semiconductor devices, by breaking down larger polymers present in the original photoresist layer into smaller, more soluble, fragments. An outer transfer unit may also be utilized for transferring the wafer and its exposed photoresist layer to a spinner developer where a developing solution may be applied to the exposed photoresist layer while the wafer rotates on a spinner, thereby developing the exposed photoresist layer to form a photoresist pattern on the wafer that exposes portions of the underlying material layer.
According to the conventional photolithographic processing described above, a major exposing process is performed in the exposing unit of the exposing apparatus such as the stepper and the scanner. However, a minor exposing process may be performed in one of the treatment units provided within the exposing apparatus. For example, a wafer edge exposing process may be carried out while the wafer is supported on a spinner or chuck provided in the treatment unit. Wafer edge exposing processes are performed for removing residual portions of the photoresist layer that would otherwise remain on the edge portions of the exposed wafer, thereby reducing the likelihood of particles being generated from residual photoresist and improving device yield.
According to a conventional method for removing residual photoresist, an additional light source is provided in the treatment unit for performing a second exposing process on the edge portions of the wafer subsequent to the first exposing process but before the developing process. In this way, the photoresist in the peripheral regions of the wafer will be removed as the photoresist pattern is formed. Because the semiconductor devices being formed on the majority of the wafer surface are typically highly integrated, a relatively expensive and short wavelength light, for example an argon fluoride excimer laser or a krypton fluoride excimer laser, is typically utilized for the first exposing process in order to accurately form the fine patterns necessary to achieve the high degree of integration.
However, because the resolution requirements for the peripheral exposure process are typically much less stringent, the relatively expensive light source used in the first exposing process is not required for satisfactory completion of the second exposing process. Accordingly, a less expensive light source, for example an extra-high pressure mercury lamp, can be utilized in the treatment unit of the exposing apparatus in combination with elements provided in a lens system or lamp housing capable of shifting the basic wavelength of the less expensive light source to provide exposure light having one or more wavelength(s) that corresponds more closely to the wavelength(s) of the more expensive light source being utilized for the first exposing process.
However, obtaining light having a sufficiently short wavelength from less expensive light sources, for example mercury lights, can necessitate maintaining a relatively high output from the mercury lamp that tends to reduce the life span of the lamp and decrease the lamp efficiency. In addition, while the first exposing process typically requires a light source having a wavelength no more than about 185 nm for satisfactorily patterning the photoresist, extra-high pressure mercury lamps, which typically emit light having a wavelength range between about 185 nm and about 2,000 nm, cannot be utilized for the first exposing process.
One method that has been suggested for addressing these problems is a frequency doubling method for transforming the light energy emitted from the mercury lamp to light energy having a sufficiently short wavelength. Difficulties, however, particularly the relatively low beam intensity of the transformed mercury light and the complexity of the apparatus required for practicing the frequency doubling method, have prevented such a process from being implemented in a production environment.
Further, utilizing an additional light source for the second exposing process introduces other complications as well including, for example, the need for a separate aligning process for the treatment unit and the chance that an overlap associated with the first exposing process will correspond precisely to the overlap associated with the second exposing process. Furthermore, the additional aligning process required for operating the treatment unit represents additional maintenance time and expense, thereby reducing the overall productivity of the exposing apparatus.
For at least these reasons, it has been suggested that the second exposing process can be performed using the light generated for the first exposing process. An example of this approach was provided in Korean Patent Laid-Open Publication No. 1999-017136, wherein the light used for the first exposing process in the exposing unit is reflected into the treatment unit and there utilized as the light for the second exposing process.
In this manner, the light used in the second exposing process conducted in the treatment unit is from the same light source, and will have substantially the same wavelength, as the light used in the first exposing process conducted in the exposing unit. Accordingly, the wafer edge exposing process may be performed without necessitating additional processes such as aligning processes for removing photomasks as well as the additional assemblies that would be required. However, because the first and second exposing processes are still conducted sequentially, the improvement in overall throughput associated with this approach, if any, is relatively modest.